Patent Document 1 discloses in the Examples that monoallyl isocyanurate and triethoxysilane are reacted to obtain an isocyanuric acid derivative of formula (E-2):

and the obtained isocyanuric acid derivative, tetraethoxysilane, methyltriethoxysilane, and phenyltrimethoxysilane are used to obtain a polymer of formula (3-5):

and further discloses that a resist underlayer film-forming composition is prepared using the obtained polymer. Patent Document 1, however, does not disclose an isocyanuric acid derivative having two alkoxyalkyl groups.